My favorite is Pulp Fiction, but the best is certainly Django Unchained. Jamie Foxx. I absolutely HATE Django Unchained, mind you; it fills me with violent furor towards the powers that be, and I don't like to be angry. But it's an amazing film.
Generally, I'd say he's super overrated, but he also hasn't made a film I've seen and disliked. I was really underwhelmed by Reservoir Dogs, but I'd still probably give it three stars. I haven't seen Death Proof, and that's the popular caveat, so it's possible it's no good. He's no Paul Thomas Anderson (and he'd probably agree with that statement) but he's made two or three great films. The rest are all at least fine.
In my list of working directors, I'd probably put him below folks like both popular Andersons (I'd choose Boogie Nights and The Life Aquatic,) the Coens (Inside Llewyn Davis,) Christopher Nolan (who can pick? Maybe Inception) and Terrence Malick (impossible not to choose The Tree of Life) before him. I'd need to see a couple more films by Darren Aranofsky (Black Swan) and Nicolas Winding Refn (I love Bronson, but Drive), and there are a handful of fantastic up-and-coming directors that come to mind.
That "greatest working directors" question is a lot tougher now that Hayao Miyazaki is supposedly done and guys like Martin Scorsese (picking Goodfellas, picking on Hugo and The Wolf of Wall Street) and David Fincher (his best is probably still Fight Club, and what a disappointment Gone Girl turned out to be) have fallen off.